This invention relates to an electronic color printing system and, more particularly, to a color printing system which utilizes a linear optical image bar to form images with improved registration on a photoreceptor surface.
Electronic color printing systems are known in the art. For example, U.S. Pat. No. 4,403,848, assigned to the same assignee as the present invention, discloses a system which uses a modulated laser ROS (raster output scanner) to form separate exposures at different locations of a photoreceptor belt, each exposure representing a specific color. Each latent image is exposed with a characteristic color toner and transferred sequentially to a copy sheet. This system shown in FIGS. 1 and 2 of the patent can be characterized as a single pass, multiple image system. FIG. 3 of the same patent shows another color printing embodiment wherein three lasers are modulated in accordance with scanned document information and three imaging beams are scanned across the moving charged photoreceptor creating three color separation images which are developed in succession to form a composite color image. This color image is thereafter transferred to a copy sheet and fused to provide a color copy. For each of the two embodiments disclosed in this patent, the color images are formed by directing the modulated laser output to the surfaces of a rotating polygon which then reflects (scans) the image across the photoreceptor surface. The major problem with this type of prior art color printing system is the difficulty in registering the color images in both the single pass and multiple pass modes. The rotating mass of the polygon presents the main difficulty in maintaining precise registration since it is subject to mechanical speed irregularities and vibration. U.S. Pat. No. 4,370,047 also assigned to the same assignee as the present invention, is directed to a color printing system which utilizes a single pass, multiple image, system. This system is subject to the same type of registration problems discussed above.
Optical image bars, as known in the art, comprise an array of optical picture elements (pixels), for converting a spatial pattern, usually represented by the information content of electrical input data signals, into a corresponding optical intensity profile. Although there are a variety of applications for these image bars in a number of different fields, a significant portion of the effort and expense that have been directed to their development has been directed towards their application to electrophotographic printing where they are relatively low cost, and a reliable alternative to the flying spot raster scanners of the type disclosed in the above discussed patents. The optical image bar generates line-like image ray patterns, representative of the image signal input, on the surface of a photoreceptor. One example of an image bar type of printer is found in U.S. Pat. No. 4,477,175, where the image bar is composed of a plurality of individually activated illumination elements (LEDs). Another example is found in U.S. Pat. No. 4,389,659 where the image bar is composed of a plurality of individually activated electro-optic electrodes. Also known as an EO TIR (electro-optic total internal reflection) spatial light monitor, this type of device characteristically comprises a plurality of laterally separated, individually addressable electrodes which are maintained on, or closely adjacent to, a reflective surface of an optically transparent electro-optic element, such as a lithium niobate (LiNbO.sub.3) crystal. In operation, substantially the full width of the electro-optic element of such a modulator is illuminated by a linearly polarized, transversely collimated light beam. Thus, when voltages representing the pixels of a linear pixel pattern (e.g., the pixels for a given line of an image) are applied to its individually addressable electrodes, the modulator spatially phase modulates the wavefront of the light beam in accordance with the applied pixel pattern. As a general rule, of course, the spatial wavefront modulation varies as a function of time in accordance with the pixel patterns for successive lines of a two dimensional image, thereby providing a line-by-line representation of the image.
According to one aspect of the present invention, a plurality of optical image bars are used in a single pass multiple image color printing system where individual laser sources are simultaneously pulsed in a timed relationship with input data modulation to form line exposures at separate photoreceptors which are in exact registration with each other. As a further aspect of the present invention, the delayed laser trigger pulses are generated in time intervals between each successive line exposure thus enabling high speed operation. The concept is equally valid to multiple pass, single image systems with the delayed pulse being simultaneously applied as two or more laser pulses coincident with data input modulation. More particularly, the invention rleates to a printer comprising an image writing section for forming line images on the surface of a photosensitive medium located in a xerographic processing section, the image writing section including at least one light source for emitting beams of output radiation of a first duration upon application of a strobe pulse thereto; modulator means in the path of said output radiation for directing a modulated light beam signal to said photoreceptor medium; means for applying data signals to said modulator, the application of said data signals being separated by a time period of a second duration, said radiation output of first duration being narrow in width compared to the time period of said second duration, and control means for controlling the application of said strobe pulses to said light source, the application occurring during said second time duration.